In principle there are two main types of wind turbines in view of the drive configuration of a wind turbine. The first type of a wind turbine is the more classical type of a wind turbine comprising a gearbox arranged between the main shaft and a generator of the wind turbine. The second type of a wind turbine is a gearless type comprising a direct drive or a directly driven generator. Such a direct drive generator can be made as a synchronous generator with winded rotor or with permanent magnets attached to the rotor, or it can be designed as an alternative type of a generator. Independently from the type of a direct drive generator it is desirable that the width of the air gap between the rotor and the stator of the generator is preferably maintained constantly or at least within certain tolerances during the operation of the wind turbine and the direct drive generator respectively even when the arrangement of wind turbine rotor, main shaft and direct drive generator is subject to loads.
Therefore different bearing arrangements for a drive configuration of a wind turbine comprising a direct drive generator were developed. Up to now the classical bearing arrangement of a direct drive generator is a two-bearing arrangement. Thereby the rotor of the generator which is connected to the wind turbine rotor is supported with two bearings towards a stationary inner shaft or a fixed inner shaft. The stator is on one side attached to the stationary inner shaft. Thus the rotor can turn relatively to the stator around the stationary inner shaft. Wind turbines with such a design are e.g. described in EP 1 641 102 A1 and U.S. Pat. No. 6,483,199 B2. The drawback of such a design is that the one-side support of the stator makes it difficult to maintain the width of the air gap at least substantially constant at the unsupported side of the stator in particular when the entire generator structure is not only subject to gravity and mass inertia but also to unbalanced magnetic pull. In order to reduce this drawback a direct drive generator with such a two-bearing arrangement needs a large and heavy stator support structure capable of absorbing relatively large bending moments of the stator. Such a stator structure is e.g. described in WO 02/05408 A1 wherein the stator structure comprises a support construction having a plenty of support arms.
In an alternative design the two-bearing arrangement is replaced with a single bearing with a stationary inner bearing part attached to a stationary inner shaft and a rotating outer bearing part supporting the rotor of the direct drive generator. Wind turbines comprising a direct drive generator with a single bearing are disclosed in US 2006/0152014 A1 and WO 02/057624 A1. But the replacement of the two bearings with a single bearing does not substantially change the drawback of the unilaterally supported stator structure.
In some further solutions the stationary inner shaft concept is replaced with a rotating shaft concept. Since the stator of the generator is supported on both sides according to the rotating shaft concept, it is easier to maintain the width of the air gap between the rotor and the stator of the generator at least substantially constantly. There are two known variants of the rotating shaft concept, one with a two-bearing arrangement and one with a four-bearing arrangement.
According to the two-bearing arrangement the bearings of the generator act as bearings of a main shaft for the wind turbine which main shaft is connected to the wind turbine rotor. The stator structure is supported towards the main shaft and attached to a bedplate of the wind turbine. Wind turbines having such a design are disclosed in U.S. Pat. No. 7,119,453 B2 and WO 03/023943 A2. A drawback of this design is that the stator structure needs to be dimensioned to absorb and transfer all wind turbine rotor loads, i.e. the weight of the wind turbine rotor and all asymmetric aerodynamic loads to maintain the width of the air gap within the necessary tolerances. On large wind turbines this leads to very heavy and expensive stator structures.
In the four-bearing arrangement the main shaft of the wind turbine which is connected to the wind turbine rotor on its one end is supported by its own two bearings and carries at its other end the direct drive generator. The direct drive generator has a two-bearing arrangement for the centering of the rotor inside the stator. An example of such a wind turbine is described in U.S. Pat. No. 6,781,276 B1. In this main shaft mounted arrangement the generator stator is carried by the generator rotor and torque is transferred from the generator to the wind turbine bedplate through a torque arm arrangement. The torque arm arrangement needs to comprise some kind of flexibility, e.g. implemented with rubber elements, to allow for minor misalignments between the main shaft-generator structure and the turbine bedplate. The bilateral support of the stator on the rotor enables for a relatively lightweight stator structure. The main drawback of this design is that a total of four bearings are required, and that the full torque has at least partially to pass through these bearings. For large wind turbines this means comparatively large and expensive bearings. Furthermore, for large wind turbines the torque arm arrangement becomes a comparatively substantial and heavy structure.